The present invention refers to optoceramics and refractive, transmissive or diffractive optical elements manufactured thereof their use as well as imaging optics. These optoceramics and optical elements are permeable to visible light and/or infrared radiation. The optoceramics consist of a crystal network, i.e. of polycrystalline material.
An optoceramic according to the present invention is understood to be highly transparent material that is essentially single phase, polycrystalline and based on an oxide. Optoceramics are thus a special subdivision of ceramics. “Single phase” in this context is understood to mean that at least more than 95% by weight of the material, preferably at least 97% by weight, further preferred 99% by weight and most preferred 99.5 to 99.9% by weight of the material are present in the form of crystals of the desired composition. The individual crystals are arranged densely and, relative to the theoretical density, they have densities of at least 99% by weight, preferably at least 99.9% by weight, further preferred at least 99.99% by weight. Accordingly, the optoceramics are nearly free of pores. The use in imaging optics refers mainly to the use of the optoceramics according to the present invention in shapes that have curved surfaces at the entry and exit position of light, i.e. preferably lens shapes.
Optoceramics are distinguished from conventional ceramics by the fact that conventional ceramics comprise high proportions of amorphous glass phase next to the crystalline phase. Similarly, within conventional ceramics the high densities of optoceramics may not be achieved. Neither glass ceramics nor conventional ceramics may exhibit the advantageous properties of optoceramics like certain refractive indexes, Abbe numbers, values for relative partial dispersion and particularly the advantageous high transparency for light in the visible and/or infrared spectral range.
“Transparency in the visible spectral range” in context of the present invention is supposed to represent a pure transmission (i.e. the transmission less reflection losses) within a range of at least 200 nm width, for example a range from 400 to 600 nm, a range of from 450 to 750 nm or preferably a range from 600 to 800 nm, in the visible light region with wavelengths of 380 nm to 800 nm of above 70%, preferably of >80%, further preferred of >90%, particularly preferred of >95% at a layer thickness of 2 mm, preferably even at a layer thickness of 3 mm, particularly preferred at a layer thickness of 5 mm.
Pure transmission in % above a certain percentage means to be a percentage based on the pure transmission that can theoretically be achieved, i.e. no reflection loss at all.
“Transparency in the infrared spectral range” in context of the present invention is supposed to represent a pure transmission (i.e. the transmission less reflection losses) within a range of at least 1000 nm width, for example a range of from 1000 to 2000 nm, a range of from 1500 to 2500 nm or further preferred a range of from 3000 to 4000 nm, in the infrared spectral range with wavelengths of from 800 nm to 5000 nm of above 70%, preferably of >80%, further preferred of >90%, particularly preferred of >95% at a layer thickness of 2 mm, preferably even at a layer thickness of 3 mm, particularly preferred at a layer thickness of 5 mm.
Ideally, the material exhibits a transmission (including reflection losses) of more than 20% at a layer thickness of 3 mm and in a wavelength range of more than 200 nm width between 5000 nm and 8000 nm.
The optical elements obtainable from the optoceramics as described herein are particularly suitable for use in imaging optics like for example objectives with reduced colour aberrations, especially with approximately apochromatic imaging properties. The optical elements manufactured from optoceramics according to the present invention are usable within lens systems in association with lenses of glass and other ceramic lenses as well, especially also in digital cameras, mobile phone cameras, in the field of microscopy, microlithography, optical data storage or other applications in the sectors of consumer and industry applications.
The main target in the development of imaging optics is a sufficient optical quality while maintaining compact and preferably light construction of the optical system. Especially for applications in digital image capturing within electronic devices like for example digital cameras, objectives in mobile phones and the like, the optical imaging system must be very small and light. In other words, the total number of imaging lenses must be kept as low as possible.
In the area of microscopy nearly diffraction limited imaging optics are needed for the ocular as well as the objective.
For the sector of military defense transparent optical systems are needed, which preferably show high transmissions in the visible wavelength region (380 to 800 nm) as well as in the infrared up to 8000 nm, ideally up to 10000 nm. Furthermore these optical systems must be resistant to external attack like mechanical influence, like for example collision, temperature, change in temperature, pressure etc.
For many other technologies like for example digital projection and further display technologies also highly transparent material is needed. But also in mainly monochromatic applications like optical storage technologies compact systems can be realized by application of material with high refractive index.
Today, development of imaging optics is limited by optical parameters of the available material. With the glass melting and moulding techniques, which are available today, only such glass types can be produced with high quality that are located in an Abbe diagram, in which the refractive index is plotted against the Abbe number, underneath a line that runs through the points Abbe number=80/refractive index=1.7 and Abbe number=10/refractive index=2.0. More precisely, glasses with refractive indices between about 1.9 and about 2.2 and an Abbe number in the range of from about 30 to about 40 tend to be unstable so that it is very difficult to manufacture such glasses in high amounts and sufficient quality. Similarly, glasses with refractive indices of between about 1.8 and 2.1 and an Abbe number in the range of from about 30 and 55 tend to be unstable.
Next to refractive index and Abbe number the relative partial dispersion is also important when choosing optical material. If one intends to produce nearly apochromatic optical systems, the combination of material with almost equal relative partial dispersion but a big difference in Abbe number becomes necessary. If the partial dispersion Pg,F is plotted against Abbe number (FIG. 2b), most glasses lie on a line (“normal line”). Desirable are hence materials, in which the combination of Abbe number and relative partial dispersion deviates from this behaviour.
The definitions of refractive index nd, Abbe number νd and relative partial dispersion Pg,F are known to the person skilled in the art and can be understood by studying the relevant technical literature. In the sense of the present invention the expressions are used according to the definitions in “The properties of optical glass; Bach, Hans; Neuroth, Norbert (Hrsg.), Berlin (u.a.): Springer, 1995.—(Schott series on glass and glass ceramics: science, technology, and applications; 1), XVII, 410 p.-2., corr. Print., 1998, XVII, 414 S”.
Material that is located above the line in the Abbe diagram mentioned before, are at this time only single crystals or polycrystalline material. The production of single crystals with the known crystal breeding techniques, however, is very expensive especially for high melting components, because of the very expensive breeding crucible material; further this method is subject to limitations with respect to chemical compositions. Furthermore, crystals cannot be produced in a near-netshape or near-netformat manner, resulting in significant postprocessing effort.
R2Ti2O7 single crystals can show high refractive indices (see Shcherbakova et al., Russ. Chem. Rev. 48, 423 (1979)). The production of single crystals is, as indicated above, very expensive and does not render manufacture of larger optical elements possible. It must be said here that the data for polycrystalline materials in the article of Shcherbakova only refer to values of micro hardness. The article K. N. Portnoi et al., Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, Vol. 6, No. 1, 91 (1970) does not contain any data or hints on refractive indices of polycrystalline material.
The crystals that are for example described in Malkin et al. Phys. Rev. B 70, 075112 (2004) are made from Yb2Ti2O7 and are obtainable via Floating Zone Methods in large individuals. The thicknesses are indicated as being up to 1.5 mm.
Although polycrystalline ceramics are obtainable in a wide range of compositions, they usually show insufficient optical quality, especially as far as homogeneity of refractive index and transmission is concerned. By now only few composition ranges and structure types are known that provide for transparent ceramics with sufficient optical quality.
For example the Japanese laid-open patent application JP 2000-203933 discloses the production of polycrystalline YAG by application of a certain sintering process. Disadvantages of YAG for passive linear optical applications are the position in the Abbe diagram or Pg,F-diagram (nd=1.83, Abbe number=52.8; Pg,F=0.558; delta Pg,F=0.0031), which is not “exotically” enough and does not suffice for most applications. The YAG system as such us furthermore disadvantageous, because although chemical variability is high, the structure only accepts trivalent cations. The possibilities of variation (tuning) of the optical properties, which are besides other factors influenced by the UV band gap structure, are therefore not sufficient for many purposes.
In the U.S. Pat. No. 6,908,872 a translucent ceramic material is described, which utilizes barium oxide as obligatory oxide in the ceramic. The thus obtained ceramics show Perovskite structure and are para-electric. However, ceramics comprising such barium containing phases with perovskite structure often show insufficient optical imaging quality. This results from the tendency of many perovskites to build out distorted ferro-electrical crystal structures and thus loose their optical isotropy. This leads inter alia to an undesired birefringence of the crystals, from which the ceramic is made. Furthermore, transmission in the blue spectral region (wavelength around 380 nm) is insufficient.
Transparent ceramics of the composition La2Hf2O7 (LHO) are known from Ji et al., “Fabrication of transparent La2Hf2O7 ceramics from combustion synthesized powders”, Mat. Res. Bull. 40 (3) 553-559 (2005)”. Therein powders of the target composition are used that have been obtained by combustion reactions. Only such ceramics are obtained that show transparencies of 70% at sample thicknesses of <1 mm, which is too little for optical applications. Ti4+-comprising, active La2Hf2O7 as transparent ceramic material for scintillator applications is known from Ji et al., “La2Hf2O7:Ti4+ ceramic scintillator for x-ray imaging” J. Mater. Res., Vol. 20 (3) 567-570 (2005) as well as from CN 1 587 196A. 0.5 at %-5 at % Tb3+-doped active LHO as transparent ceramic material is described in Ji, Y M; Jiang, D Y; Shi, J L in “Preparation and spectroscopic properties of La2Hf2O7:Tb” (MATERIALS LETTERS, 59 (8-9): 868-871 APR 2005). These active, i.e. light emitting, lanthanum compounds are not suitable for the desired application as passive, i.e. not light emitting, elements, i.e. in the form of lenses.
DE 10 2006 045 072 A1 describes an optical element including a single phase optoceramic. However, the materials are of cubic structure of the ZrO2 type which is stabilized by Y2O3. Such crystal structures are different from a stable cubic pyrochlore or stable fluorite structure.
Klimin et al in “Physics of solid state. Vol. 47, No. 8, 2005” describes single crystal materials and polycrystalline compounds, however does not address optical grade transparency of any polycrystalline material. Polycrystalline material is in the form of pressed powder that is slightly consolidated at quite low temperatures not higher than 1400° C. This procedure cannot result in a material with optical properties.
WO 2007/060816, which was published after the priority date of the present invention addresses translucent ceramics. A refractive optical element in the present invention is understood to be an optical element, in which electromagnetic rays are refracted at the interface to the optical element, because of the property of the optical element to consist of an optically thinner or optically thicker material than the vicinity. Preferably, the present description refers to optically refractive elements that behave “imaging refractive”, i.e. they comprise an entry and an exit surface of the optically denser body that is curved in a lens shape. Thus deflection of the light ray after passing the element is achieved.